1. Field of the Invention
The present invention relates to a litz wire and a coil for a high frequency application.
2. Description of the Prior Art
Generally, when a high frequency current flows through a conductor, there occurs a phenomenon that the high frequency current is limited to a surface of the conductor and does not enter the interior of the conductor. This phenomenon is called the "skin effect". Since the high frequency current flows through only the surface portion of the conductor, the effective resistance of the conductor is increased. The above-mentioned increase in the effective resistance is remarkable when the diameter of the conductor is nearly equal to or greater than the skin depth. In order to avoid such an increase in effective resistance, a ribbon-like conductor is used, or thin insulated wires are twisted into a wire bundle so that the high frequency current is distributed uniformly at the cross section of the wire bundle. Such a wire bundle is called the "litz wire".
On the other hand, a magnetic bubble memory includes a high frequency current coil. The magnetic bubble memory is provided with a rotating magnetic field-generating circuit for transferring bubble domains, and the coil is used as a constituent element of the rotating field generating circuit. As is well-known, the rotating field is generated by arranging a plurality of coils, for example, two coils so that magnetic fields generated by the coils perpendicularly intersect each other, and by causing currents having a .pi./2 phase difference with respect to each other to flow through the coils. These coils are usually driven at a frequency of 50 to 100 kHz.
In order to write in and read out data at high speed in the above-mentioned magnetic bubble memory by the increasing requirement of mass memory, it is required to drive the coils at higher frequencies. Accordingly it is necessary to take measures sufficient to solve the problems with respect to the skin effect.
A conventional litz wire is formed in such a manner that seven elementary solid wires (hereinafter referred to as "strands") are employed and that six strands are twisted around one strand (that is, concentric twisting is performed), in order for the circumference of the completed wire to have a circular form. In other words, the strand disposed on the center line of the litz wire is extended substantially in the form of a straight line, and six surrounding strands are helically twisted so as to enclose the center strand.
Now, let us consider the skin effect in a litz wire. The skin effect at each strand is affected not only by a magnetic field due to its own current but also by a magnetic field due to a current flowing through adjacent strands. As a result, in the conventional litz wire, the six surrounding strands are approximately equal in current density to each other, since these strands are arranged alike in the litz wire. However, the strand disposed on the center is strongly affected by the skin effect as compared with the surrounding strands, and is smaller in current density than the surrounding strands. In other words, since the seven strands are unequal in arrangement, the current density distribution of each of the strands is not uniform, and thus the loss resistance of the litz wire is increased.
As mentioned above, the current density of each strand is not constant in the conventional litz wire of concentric twisting. Therefore, the conventional litz wire fails to completely solve the problems with respect to the skin effect.